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CRAT practice questions

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Question 1Rhythm Analysis

On a standard ECG rhythm strip, which interval represents the time from the beginning of atrial depolarization to the beginning of ventricular depolarization?

  1. PR interval Correct answer.
  2. RR interval
  3. ST segment
  4. QT interval

Explanation

The PR interval is measured from the onset of the P wave to the onset of the QRS complex and reflects conduction through the atria and the AV node. The normal range is roughly 0.12 to 0.20 seconds, and prolongation beyond this suggests first-degree AV block.

Question 2Rhythm Analysis

A rhythm strip shows a regular narrow-complex rhythm at 75 beats per minute with a P wave before every QRS and a constant PR interval. What is the most appropriate interpretation?

  1. Atrial fibrillation
  2. Third-degree AV block
  3. Normal sinus rhythm Correct answer.
  4. Ventricular tachycardia

Explanation

A regular rate between 60 and 100 per minute with consistent P waves preceding each QRS and a stable PR interval defines normal sinus rhythm. The narrow QRS confirms conduction through the normal His-Purkinje system.

Question 3Rhythm Analysis

How is the ventricular rate estimated from an ECG when the rhythm is regular and standard 25 mm/s paper is used?

  1. Divide 300 by the number of large boxes between R waves Correct answer.
  2. Divide 1500 by the number of small boxes between P waves
  3. Count the total QRS complexes on the strip and multiply by 10
  4. Multiply the number of large boxes between R waves by 300

Explanation

At 25 mm/s, one large box equals 0.2 seconds, so 60 seconds divided by 0.2 gives 300 large boxes per minute. Dividing 300 by the large boxes between consecutive R waves yields the rate for a regular rhythm.

Question 4Rhythm Analysis

Which feature best distinguishes a premature atrial complex from a premature ventricular complex on a rhythm strip?

  1. The premature beat is wider than the preceding sinus complexes
  2. The premature beat is preceded by an ectopic P wave and has a narrow QRS Correct answer.
  3. The premature beat is followed by a fully compensatory pause
  4. The premature beat occurs after a progressively lengthening PR interval

Explanation

A premature atrial complex arises above the ventricles, so it typically shows an early, abnormally shaped P wave followed by a narrow QRS conducted through the normal pathway. A premature ventricular complex originates in ventricular myocardium and produces a wide, bizarre QRS without a preceding atrial P wave.

Question 5Rhythm Analysis

A rhythm shows a progressively lengthening PR interval followed by a dropped QRS complex, with the RR interval shortening before the pause. Which conduction disturbance is present?

  1. Mobitz type II second-degree AV block
  2. Complete AV block
  3. First-degree AV block
  4. Mobitz type I second-degree AV block Correct answer.

Explanation

Mobitz type I, or Wenckebach, is characterized by progressive PR prolongation until a QRS is dropped, after which the cycle resets. The shortening RR intervals before the pause result from the decreasing increment of AV delay.

Question 6Rhythm Analysis

Which finding is most consistent with atrial flutter rather than atrial fibrillation?

  1. Narrow QRS complexes with a chaotic atrial baseline
  2. Irregularly irregular ventricular response with no organized atrial activity
  3. Regular sawtooth flutter waves at about 300 per minute Correct answer.
  4. Absence of P waves with a fine undulating baseline

Explanation

Atrial flutter is produced by a macroreentrant circuit, typically in the right atrium, generating regular sawtooth waves at roughly 250 to 350 per minute. Atrial fibrillation instead shows disorganized, chaotic atrial activity without discrete organized waves.

Question 7Rhythm Analysis

A wide-complex tachycardia is regular at 160 beats per minute and shows no visible P waves. What is the safest initial rhythm interpretation for clinical management?

  1. Treat as atrial fibrillation with rapid ventricular response
  2. Treat as ventricular tachycardia until proven otherwise Correct answer.
  3. Treat as sinus tachycardia with aberrancy
  4. Treat as a junctional escape rhythm

Explanation

A regular wide-complex tachycardia should be presumed ventricular tachycardia until proven otherwise, because mislabeling it as supraventricular can lead to harmful treatment. Ventricular tachycardia is the most common cause of wide-complex tachycardia, especially with structural heart disease.

Question 8Rhythm Analysis

Which ECG change is most characteristic of hyperkalemia and can progress to a sine-wave pattern?

  1. Prolonged QT interval with alternating T-wave amplitude
  2. Prominent U waves with flattened T waves
  3. Diffuse ST elevation with PR depression
  4. Peaked, narrow-based T waves Correct answer.

Explanation

Hyperkalemia initially produces tall, peaked, narrow-based T waves as repolarization is accelerated, and higher levels widen the QRS and can produce a sine-wave morphology. These changes are a warning of impending ventricular arrhythmia.

Question 9Rhythm Analysis

A rhythm strip shows an irregularly irregular narrow-complex rhythm at about 130 beats per minute with no discernible P waves. Which rhythm is most likely?

  1. Atrial fibrillation with rapid ventricular response Correct answer.
  2. Sinus tachycardia
  3. Multifocal atrial tachycardia
  4. Atrial flutter with 2:1 conduction

Explanation

An irregularly irregular rhythm without discrete P waves is the defining feature of atrial fibrillation, and a rate above 100 indicates a rapid ventricular response. The narrow QRS confirms supraventricular conduction.

Question 10Rhythm Analysis

What does the QT interval corrected for heart rate (QTc) estimate?

  1. The duration of ventricular depolarization and repolarization adjusted to a rate of 60 per minute Correct answer.
  2. The time required for the atria to repolarize
  3. The conduction delay across the AV node
  4. The duration of the ventricular action potential plateau only

Explanation

The QT interval measures total ventricular electrical activity, from depolarization through repolarization, and varies with heart rate. Correcting it to a standard rate of 60 per minute allows comparison across rates and helps identify patients at risk for torsades de pointes.

Question 11Rhythm Analysis

A patient has a heart rate of 45 per minute with a narrow QRS, a P wave before every QRS, and a normal PR interval. What is the correct interpretation?

  1. Sinus bradycardia Correct answer.
  2. Ventricular escape rhythm
  3. Complete heart block
  4. Junctional rhythm

Explanation

A rate below 60 per minute with a normal P wave preceding each QRS and a normal PR interval defines sinus bradycardia. The sinus node remains the pacemaker, so the QRS stays narrow.

Question 12Rhythm Analysis

Which rhythm is characterized by a completely irregular ventricular response and an absence of organized atrial activity, requiring anticoagulation risk assessment?

  1. Sinus arrhythmia
  2. Wandering atrial pacemaker
  3. Atrial flutter with variable block
  4. Atrial fibrillation Correct answer.

Explanation

Atrial fibrillation produces an irregularly irregular ventricular response because the AV node conducts chaotic atrial impulses unpredictably. Because of the associated thromboembolic risk, stroke risk stratification and anticoagulation decisions are central to management.

Question 13Rhythm Analysis

What is the defining ECG feature of first-degree AV block?

  1. A progressively lengthening PR interval with dropped beats
  2. A PR interval greater than 0.20 seconds with every P wave conducted Correct answer.
  3. Complete dissociation of P waves and QRS complexes
  4. A constant PR interval with occasional dropped QRS complexes

Explanation

First-degree AV block is simply a prolonged PR interval above 0.20 seconds, with every atrial impulse still reaching the ventricles. It is usually benign and does not require pacing.

Question 14Rhythm Analysis

Which finding on a rhythm strip suggests a ventricular origin of an ectopic beat?

  1. A narrow QRS complex preceded by an abnormal P wave
  2. A premature beat followed by a noncompensatory pause
  3. A wide QRS complex without a preceding P wave Correct answer.
  4. A QRS complex that is identical to the sinus complexes

Explanation

Ectopic beats arising in ventricular myocardium spread cell-to-cell rather than through the fast His-Purkinje system, producing a wide QRS. Because the impulse does not originate in the atria, no preceding P wave is seen.

Question 15Rhythm Analysis

Which medication effect is most strongly associated with QT prolongation and torsades de pointes?

  1. Class IA and class III antiarrhythmic drugs Correct answer.
  2. Loop diuretics causing hypokalemia
  3. Calcium channel blockers
  4. Beta-blockers

Explanation

Class IA and class III antiarrhythmic agents prolong repolarization and are well-recognized causes of QT prolongation and torsades de pointes. Risk increases with bradycardia, hypokalemia, hypomagnesemia, and concurrent QT-prolonging drugs.

Question 16Rhythm Analysis

A rhythm shows a rate of about 150 per minute, regular, with narrow QRS complexes and a sawtooth baseline. What is the most likely rhythm?

  1. Ventricular tachycardia
  2. Sinus tachycardia
  3. Atrial flutter with 2:1 conduction Correct answer.
  4. Atrial fibrillation

Explanation

Atrial flutter typically has an atrial rate near 300 per minute, and 2:1 AV conduction yields a regular ventricular rate near 150. The sawtooth baseline reflects the organized macroreentrant flutter circuit.

Question 17Rhythm Analysis

Which statement about the sinus node is correct?

  1. It is the dominant pacemaker with an intrinsic rate of 60 to 100 per minute Correct answer.
  2. It has an intrinsic rate of 40 to 60 per minute and is a backup pacemaker
  3. It depolarizes only when stimulated by the vagus nerve
  4. It conducts impulses directly to the ventricular myocardium

Explanation

The sinus node normally dominates because it has the fastest spontaneous depolarization rate, typically 60 to 100 per minute. Lower pacemakers are suppressed by overdrive until the sinus node fails or slows.

Question 18Rhythm Analysis

What ECG pattern is expected with hypokalemia?

  1. Shortened QT interval with tall T waves
  2. Peaked T waves with a widened QRS
  3. Diffuse ST elevation
  4. Flattened T waves with prominent U waves Correct answer.

Explanation

Hypokalemia hyperpolarizes myocardial cells and delays repolarization, producing flattened T waves, ST depression, and prominent U waves. Severe hypokalemia also predisposes to ventricular arrhythmias.

Question 19Rhythm Analysis

Which of the following best describes the mechanism of reentry in arrhythmogenesis?

  1. A circuit in which an impulse re-excites tissue after the refractory period has recovered Correct answer.
  2. Spontaneous depolarization of a pacemaker cell during diastole
  3. Delayed afterdepolarizations triggered by calcium overload
  4. An impulse generated by a single ectopic focus firing rapidly

Explanation

Reentry requires two pathways with different conduction and refractoriness, allowing an impulse to travel around a circuit and re-excite tissue that has recovered. This mechanism underlies many tachyarrhythmias, including atrial flutter and AV nodal reentrant tachycardia.

Question 20Rhythm Analysis

A rhythm strip shows a regular rate of 40 per minute with wide QRS complexes and no visible P waves. What is the most likely rhythm?

  1. Mobitz type I block
  2. Junctional escape rhythm
  3. Ventricular escape rhythm Correct answer.
  4. Sinus bradycardia

Explanation

When higher pacemakers fail, a ventricular focus can emerge with an intrinsic rate of roughly 20 to 40 per minute. Because the impulse originates below the bundle of His and spreads slowly, the QRS is wide.

Question 21Rhythm Analysis

Which finding is most consistent with multifocal atrial tachycardia?

  1. A wide QRS with no P waves
  2. A sawtooth baseline with a regular ventricular response
  3. A regular rhythm with a single P-wave morphology
  4. An irregular rhythm with at least three distinct P-wave morphologies Correct answer.

Explanation

Multifocal atrial tachycardia is defined by an irregular rhythm with at least three different P-wave shapes, reflecting multiple competing atrial foci. It is most often seen in patients with severe pulmonary disease.

Question 22Rhythm Analysis

What is the significance of a delta wave on the ECG?

  1. It reflects a ventricular escape beat
  2. It is a marker of atrial repolarization
  3. It represents delayed conduction through the AV node
  4. It indicates an accessory pathway causing ventricular pre-excitation Correct answer.

Explanation

A delta wave is a slurred upstroke of the QRS caused by an accessory pathway that bypasses the AV node and pre-excites part of the ventricle. It is the hallmark of Wolff-Parkinson-White syndrome and creates a risk of rapid conduction during atrial fibrillation.

Question 23Rhythm Analysis

During a 6-second rhythm strip, 9 QRS complexes are counted. What is the approximate heart rate?

  1. 90 beats per minute Correct answer.
  2. 54 beats per minute
  3. 60 beats per minute
  4. 150 beats per minute

Explanation

The 6-second method multiplies the number of QRS complexes in a 6-second strip by 10 to estimate the rate per minute. Nine complexes multiplied by 10 gives approximately 90 beats per minute.

Question 24Rhythm Analysis

Which conduction pattern is characteristic of third-degree AV block?

  1. Independent atrial and ventricular rhythms with no consistent PR relationship Correct answer.
  2. Intermittent dropped QRS complexes with a constant PR interval
  3. A fixed prolonged PR interval with all P waves conducted
  4. Progressive PR lengthening until a QRS is dropped

Explanation

In third-degree, or complete, AV block, no atrial impulses reach the ventricles, so the atria and ventricles beat independently. The PR interval varies randomly and there is AV dissociation, often with a slow escape rhythm.

Question 25Rhythm Analysis

Why does the QRS complex widen in bundle branch block?

  1. The sinus node fires at an accelerated rate
  2. The PR interval becomes longer than normal
  3. Atrial repolarization overlaps the QRS
  4. Ventricular activation proceeds sequentially rather than simultaneously through both bundles Correct answer.

Explanation

When one bundle branch is blocked, the affected ventricle is activated later through slower myocardial spread after the intact side depolarizes. This loss of simultaneous activation prolongs the QRS beyond 0.12 seconds.

Question 26Rhythm Analysis

On a standard ECG recorded at 25 mm/s, how many large boxes separate consecutive R waves when the ventricular rate is 60 per minute?

  1. Ten large boxes
  2. Five large boxes Correct answer.
  3. Three large boxes
  4. Six large boxes

Explanation

At 25 mm/s each large box represents 0.20 seconds, and 60 beats per minute corresponds to one beat every 1.0 second. Dividing 1.0 second by 0.20 second gives five large boxes, which is why the 300-150-100 rule begins with 300 divided by five.

Question 27Rhythm Analysis

A rhythm strip shows a QRS complex that falls on a heavy line, with the next two complexes falling one small box and then two small boxes after successive heavy lines. What is the approximate ventricular rate?

  1. About 75 per minute Correct answer.
  2. About 60 per minute
  3. About 150 per minute
  4. About 100 per minute

Explanation

When the rhythm is irregular or the interval spans heavy lines, the 300-150-100 sequence is unreliable, so the small-box method is used. Counting small boxes between two consecutive R waves and dividing 1500 by that count gives the rate; here the intersected interval corresponds to roughly 20 small boxes, giving about 75 per minute.

Question 28Rhythm Analysis

Which interval is measured from the beginning of the P wave to the beginning of the QRS complex?

  1. QT interval
  2. PR interval Correct answer.
  3. RR interval
  4. QRS duration

Explanation

The PR interval begins at the onset of atrial depolarization and ends where ventricular depolarization starts, so it reflects conduction through the atria, AV node, and His-Purkinje system. A normal PR interval is 0.12 to 0.20 seconds, and prolongation beyond that defines first-degree AV block.

Question 29Rhythm Analysis

A rhythm is regular at 88 per minute with a P wave before every QRS, a constant PR interval of 0.16 second, and a QRS of 0.09 second. Which interpretation fits these findings?

  1. Sinus tachycardia
  2. Normal sinus rhythm Correct answer.
  3. Junctional rhythm
  4. Sinus arrhythmia

Explanation

All measured values fall within accepted limits: rate 60 to 100, PR 0.12 to 0.20 second, and QRS under 0.12 second, with a consistent P wave preceding each complex. That combination identifies a normal sinus rhythm originating from the sinus node.

Question 30Rhythm Analysis

Which feature distinguishes a premature atrial complex from a premature ventricular complex on a rhythm strip?

  1. The ectopic beat shows a QRS wider than 0.12 second
  2. The ectopic beat occurs later than the next expected sinus beat
  3. The ectopic beat is preceded by an early P wave and has a narrow QRS Correct answer.
  4. The ectopic beat is followed by a fully compensatory pause

Explanation

A premature atrial complex arises above the ventricles, so it is usually preceded by an early, often differently shaped P wave and conducts through the normal His-Purkinje system, producing a narrow QRS. A premature ventricular complex originates in ventricular myocardium, so it lacks a preceding P wave and is wide and bizarre.

Question 31Rhythm Analysis

A rhythm strip shows a pattern in which the PR interval lengthens progressively over several beats until a P wave is not followed by a QRS, after which the cycle restarts. Which conduction disturbance is this?

  1. Mobitz type II second-degree AV block
  2. First-degree AV block
  3. Third-degree AV block
  4. Mobitz type I second-degree AV block Correct answer.

Explanation

Progressive PR prolongation before a dropped beat is the hallmark of Wenckebach, or Mobitz type I, block, which usually reflects conduction delay within the AV node. The grouped beating pattern with a recurring cycle distinguishes it from fixed-ratio or complete block.

Question 32Rhythm Analysis

Which statement best explains why the ventricular rhythm in third-degree AV block is usually slower than the atrial rate?

  1. The AV node conducts every other impulse as a fixed 2:1 ratio
  2. The atria fibrillate, so no effective atrial rate exists
  3. The escape pacemaker below the block fires at its own intrinsic slower rate Correct answer.
  4. The sinus node is suppressed by retrograde conduction from the ventricles

Explanation

In complete AV block no supraventricular impulses reach the ventricles, so a subsidiary pacemaker in the junction or ventricles takes over. These escape pacemakers have slower intrinsic firing rates than the sinus node, which is why the ventricular rate is slower and independent.

Question 33Rhythm Analysis

Which finding is expected when a patient has an implanted dual-chamber pacemaker pacing the ventricles?

  1. A pacing spike falling within the T wave of the preceding beat
  2. A pacing spike immediately followed by a wide QRS complex Correct answer.
  3. A pacing spike followed by a normal narrow QRS complex
  4. A pacing spike with no subsequent electrical activity

Explanation

Ventricular pacing depolarizes ventricular myocardium directly rather than through the His-Purkinje system, so the resulting QRS is wide and resembles a bundle branch block pattern. The pacing artifact precedes the captured complex by a very short interval.

Question 34Rhythm Analysis

On a rhythm strip, a run of three consecutive wide QRS complexes at 180 per minute is seen in a patient with a known prior myocardial infarction. Which feature most strongly supports a ventricular rather than supraventricular origin?

  1. A rate above 150 per minute
  2. A regular RR interval during the run
  3. A QRS duration of 0.14 second
  4. Presence of AV dissociation during the run Correct answer.

Explanation

AV dissociation during a wide-complex tachycardia is strong evidence of ventricular origin, because the atria continue to fire independently of the ventricular rhythm. A prior infarction also raises the likelihood of a reentrant ventricular circuit.

Question 35Rhythm Analysis

Which rhythm is characterized by a ventricular rate that is a fixed fraction of the atrial rate, with a constant PR interval in the conducted beats and occasional dropped QRS complexes?

  1. Mobitz type II second-degree AV block Correct answer.
  2. Wandering atrial pacemaker
  3. First-degree AV block
  4. Sinus arrest

Explanation

Mobitz type II block conducts some atrial impulses with a stable PR interval while others fail to reach the ventricles, producing dropped beats without prior PR lengthening. It often reflects disease in the His-Purkinje system and carries a higher risk of progression to complete block.

Question 36Rhythm Analysis

Which statement about the QT interval is correct when assessing a rhythm strip?

  1. It should be measured in the lead with the clearest T wave end and corrected for rate Correct answer.
  2. It is measured from the start of the P wave to the end of the T wave
  3. It is best measured from the R wave to the start of the next P wave
  4. It varies little with heart rate and needs no correction

Explanation

The QT interval is measured from the onset of the QRS to the end of the T wave, ideally in a lead where the T wave termination is clear. Because the interval shortens as rate rises, it is corrected for heart rate to give the QTc for comparison with normal limits.

Question 37Rhythm Analysis

A patient with severe hypocalcemia is monitored, and the ECG shows a markedly lengthened QT interval without other major abnormalities. Which component of the QT interval is most directly prolonged?

  1. PR segment
  2. QRS duration
  3. ST segment Correct answer.
  4. P wave duration

Explanation

Hypocalcemia prolongs the plateau phase of the ventricular action potential, which appears on the ECG as a lengthened ST segment and therefore a longer QT interval. The QRS and atrial components are not characteristically affected.

Question 38Rhythm Analysis

Which maneuver is most appropriate to help distinguish sinus tachycardia from a paroxysmal supraventricular tachycardia in a stable patient?

  1. Carotid sinus massage while monitoring the rhythm Correct answer.
  2. Immediate synchronized cardioversion
  3. Having the patient hyperventilate rapidly
  4. Administration of atropine

Explanation

Vagal maneuvers such as carotid sinus massage transiently slow AV nodal conduction and can terminate a reentrant supraventricular tachycardia or reveal flutter waves. Sinus tachycardia typically slows gradually and then reaccelerates rather than terminating abruptly.

Question 39Rhythm Analysis

Which statement about the atrial rate in atrial flutter is correct?

  1. It is typically 250 to 350 per minute with organized flutter waves Correct answer.
  2. It is typically 100 to 150 per minute with varying P wave shapes
  3. It is typically 60 to 100 per minute with upright P waves
  4. It is typically 350 to 600 per minute with chaotic fibrillatory activity

Explanation

Atrial flutter is a macroreentrant atrial rhythm with an organized circuit, producing regular flutter waves at roughly 250 to 350 per minute. The ventricles respond at a fraction of that rate, often 2:1 or 4:1, depending on AV nodal conduction.

Question 40Rhythm Analysis

Which feature on a rhythm strip most suggests that a narrow-complex tachycardia is atrial tachycardia rather than AV nodal reentrant tachycardia?

  1. A rate that is an exact multiple of the sinus rate
  2. A gradual onset and offset of the tachycardia
  3. A P wave buried within or just after the QRS with a very short RP interval
  4. A P wave with an abnormal axis preceding each QRS with a short RP interval Correct answer.

Explanation

In atrial tachycardia the impulse originates in an ectopic atrial focus, so the P wave axis differs from sinus and the P wave often precedes the QRS with a measurable RP interval. In typical AV nodal reentrant tachycardia the retrograde P wave is usually buried in or just after the QRS.

Question 41Rhythm Analysis

Which ECG finding is most consistent with a junctional escape rhythm?

  1. A narrow QRS at 60 to 100 per minute with a P wave before every complex
  2. A narrow QRS at 40 to 60 per minute with absent or inverted P waves Correct answer.
  3. A wide QRS at 20 to 40 per minute with no P waves
  4. A narrow QRS at 100 to 150 per minute with upright P waves

Explanation

A junctional escape rhythm arises in the AV junction when the sinus node fails, firing at its intrinsic rate of about 40 to 60 per minute. Because the impulse travels retrogradely to the atria, P waves may be absent, inverted, or follow the QRS, while ventricular conduction remains narrow.

Question 42Rhythm Analysis

A monitor shows a rhythm with no visible P waves, a regular narrow QRS at 150 per minute, and a baseline that is flat between complexes. Which rhythm is most likely?

  1. Sinus tachycardia
  2. Atrial fibrillation
  3. Ventricular tachycardia
  4. Atrial flutter with 2:1 conduction Correct answer.

Explanation

A regular narrow-complex tachycardia at about 150 per minute is the classic presentation of atrial flutter with 2:1 AV conduction, because the atrial rate near 300 is halved. Flutter waves may be hidden within the QRS and T wave at this ratio, leaving an apparently flat baseline.

Question 43Rhythm Analysis

Which statement about the compensatory pause after a premature ventricular complex is correct?

  1. It is absent because the sinus node is unaffected
  2. It is usually incomplete, so the sinus node is reset by the ectopic beat
  3. It is usually fully compensatory, so the interval surrounding the ectopic beat equals two sinus cycles Correct answer.
  4. It is variable and depends on the preceding PR interval

Explanation

A premature ventricular complex usually does not conduct retrogradely to reset the sinus node, so the next sinus impulse arrives on schedule. The result is a fully compensatory pause, in which the RR intervals before and after the ectopic beat together equal two normal sinus cycles.

Question 44Rhythm Analysis

Which finding on a rhythm strip is most consistent with an escape beat rather than an ectopic beat?

  1. The complex is preceded by a P wave with a shorter PR interval
  2. The complex appears later than the next expected sinus beat Correct answer.
  3. The complex appears earlier than the next expected sinus beat
  4. The complex is followed by a fully compensatory pause

Explanation

An escape beat occurs when the dominant pacemaker fails to fire on time, allowing a subsidiary pacemaker to depolarize the heart after a pause. It therefore appears later than the expected sinus beat, whereas an ectopic beat appears earlier.

Question 45Rhythm Analysis

Which statement about the sinus node is correct regarding its role in the normal cardiac cycle?

  1. It fires only when the AV node fails to conduct
  2. It is located in the left atrium near the pulmonary veins
  3. It depolarizes the ventricles directly through the His bundle
  4. It is the dominant pacemaker because it has the fastest intrinsic rate of any pacemaker Correct answer.

Explanation

The sinus node has the highest intrinsic firing rate among cardiac pacemakers, so it normally dominates and suppresses slower subsidiary pacemakers. Its location at the junction of the superior vena cava and right atrium allows it to initiate atrial depolarization that spreads toward the AV node.

Question 46Rhythm Analysis

Which feature on a rhythm strip most strongly suggests that a wide QRS complex is due to aberrant conduction rather than ventricular origin?

  1. Absence of any P wave activity before the complex
  2. A preceding P wave with a consistent PR interval and a typical bundle branch block pattern Correct answer.
  3. A QRS axis that is extremely deviated
  4. A QRS duration greater than 0.16 second

Explanation

Aberrant conduction occurs when a supraventricular impulse reaches the ventricles while one bundle branch is refractory, producing a bundle branch block pattern that follows a P wave at a stable PR interval. The morphology usually matches a recognized right or left bundle branch block pattern.

Question 47Rhythm Analysis

A patient's rhythm strip shows a rate of 40 per minute, a narrow QRS, and no visible P waves, with a QRS morphology identical to the patient's baseline sinus beats. Which mechanism best explains this rhythm?

  1. A reentrant circuit within the AV node
  2. A junctional escape pacemaker firing after sinus node failure Correct answer.
  3. Enhanced automaticity of an atrial focus
  4. A ventricular escape pacemaker firing after complete AV block

Explanation

A narrow QRS identical to baseline sinus beats indicates that ventricular activation proceeds through the normal His-Purkinje system, which points to an origin above the ventricles. A rate of 40 per minute with absent P waves fits a junctional escape rhythm triggered by sinus node failure.

Question 48Rhythm Analysis

Which statement best describes the relationship between atrial rate and ventricular rate in atrial fibrillation?

  1. Atrial activity is organized and the ventricular response is a fixed ratio
  2. Atrial activity is organized and the ventricular response is always 2:1
  3. Atrial activity is chaotic and the ventricular response is irregularly irregular Correct answer.
  4. Atrial activity is absent and the ventricular response is regular

Explanation

In atrial fibrillation, multiple disorganized atrial wavelets produce no effective atrial contraction, and AV nodal conduction is bombarded irregularly. The result is an irregularly irregular ventricular rhythm with no consistent relationship to atrial activity.

Question 49Rhythm Analysis

Which ECG change is most characteristic of hyperkalemia as it progresses toward severe cardiotoxicity?

  1. Prolonged QT interval with flat T waves
  2. Tall, peaked T waves followed by widening of the QRS complex Correct answer.
  3. Diffuse ST depression with prominent U waves
  4. Sawtooth flutter waves with 2:1 conduction

Explanation

Hyperkalemia initially produces tall, narrow, peaked T waves as repolarization is accelerated, and as potassium rises further, conduction slows and the QRS widens. Severe hyperkalemia can progress to a sine-wave pattern and cardiac arrest.

Question 50Rhythm Analysis

A patient with a known first-degree AV block develops a new Mobitz type II pattern with a wide QRS. Which clinical concern is most relevant?

  1. High risk of progression to complete AV block Correct answer.
  2. Low risk because the block is at the AV node
  3. High risk of torsades de pointes
  4. High risk of atrial fibrillation

Explanation

Mobitz type II block, especially with a wide QRS, usually reflects disease below the AV node in the His-Purkinje system, which can progress unpredictably to complete heart block. This is why pacing is often considered even in patients who are currently asymptomatic.